DESCRIPTION: Brucella spp. have several pathogenic properties that make them a serious potential threat for use as agents of biological warfare and bioterrorism. Specifically, they are highly infectious by the aerosol route, they produce a chronic, debilitating disease in humans that is difficult to treat, and there is no safe and effective vaccine available to prevent human brucellosis. Prolonged survival and replication in host macrophages is critical to the capacity of the brucellae to establish and maintain chronic infection in the host. During their long term residence in host macrophages, the brucellae encounter a variety of harsh environmental conditions including nutrient limitation and exposure to reactive oxygen intermediates and acidic pH. Experimental evidence indicates that the B. abortus hfq gene product (also known as host factor I, or HF-I) is essential for the capacity of this organism to withstand exposure to these environmental stresses in host macrophages. Based on the well documented function of its enteric counterparts, the Principal Investigator's working hypothesis is that the B. abortus hfq gene product performs this function by facilitating optimal translation of the gene encoding a homologue of the stationary phase specific RNA polymerase sigma factor RpoS. The specific aims of this project are: 1) to clone the B. abortus rpoS gene, confirm its regulatory link to HF-I, and evaluate its contribution to stationary phase physiology in vitro and virulence in the mouse model; 2) to determine if HF-I and RpoS control stationary phase expression of the B. abortus katE and sodC genes, which encode important primary antioxidants linked to virulence in mice; and 3) to identify other HF-I and RpoS-regulated genes in B. abortus that play critical roles in the capacity of this bacterium to establish and maintain chronic infection in the murine host. Defining the physiologic state of the intracellular brucellae during chronic infection in the host and elucidating the contributions of individual stationary phase gene products to successful survival and replication in host macrophages should provide important basic information regarding host-pathogen interactions in Brucella infections. This information may also be useful for the design of novel vaccine candidates and improved chemotherapeutic approaches.